June 29, 2007—Still shrouded in mystery, these "night-shining clouds" were photographed illuminating the sky over Budapest, Hungary, on June 15.

They are among the first such noctilucent clouds pictured this year.

The clouds form in the upper layer of the Earth's atmosphere in summer.

Scientists don't know much about how the clouds form, or why they have been appearing more often at lower latitudes and glowing brighter.

To find out, NASA has employed a satellite, named AIM (Aeronomy of Ice in Mesosphere) to observe two cloud seasons over both of Earth's Poles. On June 11, AIM captured its first images of night shining clouds (not shown). The satellite will also provide scientists with data on the size and shape of the particles that make up the clouds.

"It is clear that these clouds are changing, a sign that a part of our atmosphere is changing and we do not understand how, why, or what it means," AIM principal investigator James Russell III said in statement.

The clouds "could represent an early warning that our Earth environment is being changed."